In the use of fluid seals between a rotor and a stator a common situation arises where the rotor passes from a high pressure region to a low pressure region and no fluid leakage between the two regions can be accepted. This is a problem because there is always some leakage through any seal between relatively moving parts with a pressure differential across it. In order to eliminate this leakage a region at a lower pressure than either of the other two regions is placed between them and seals placed between this very low pressure region and the high and low pressure regions. As a result fluid will leak from both the high and low pressure regions into the very low pressure region from which the leakage fluid can be ejected.
FIG. 1 shows a stylized longitudinal section through such a known arrangement of brush seals.
Referring to FIG. 1, it has been found that if a rotor 1 rotates in such a way that there are gaps between the seals 2,3 and the rotor 1 it is possible for fluid denoted by the arrow 4 to travel along the surface of the rotor 1 from the high pressure region, through the lowest pressure region and into he low pressure region if the differential pressure across the seal 2 is greater than the differential pressure across the seal 3. This can be prevented by stepping the surface of the rotor 1 so that the radius of the rotor is greater at the seal 3 than at the seal 2 as shown in FIG. 1A.
This is a problem because the relative radii of the rotor at the two seals is often fixed by the design of the apparatus including the rotor and stator, so it may be undesirable or impossible to have the rotor radius at the low pressure seal greater than that at the high pressure seal, making it impossible to prevent leakage across the seals.
This invention is intended to produce a brush seal arrangement at least partially overcoming this problem.